It is well known to produce flat articles in particular slabs using mixes obtained by mixing stone or ceramic granules with binders. The starting stone materials may be natural hard stone and the binders may be organic, chosen from among synthetic resins, or inorganic, for example of the cement based type. The production envisages the use of supports slightly bigger than the slabs, which may have large dimensions, for example 3.20×1.60 m, and very small thicknesses, for example 8 mm. After distribution of the mix in a thin layer, the article firstly undergoes a vibrocompaction step, namely pressing and simultaneous vibration, in a vacuum environment, and then a hardening step, which requires a heat supply when the binder is organic. Therefore taking into account:                the numerous compositions of the mixes considered here;        the fact that the mixes are sticky and have a consistency very similar to that of wet sand and therefore markedly different from the consistency of a concrete;        the articles produced with the mixes are intended for use either in the building sector, as flooring and cladding for internal or external walls of residential buildings, or in the furnishing sector, as work-tops for kitchens or bathrooms;        it is often required to use liquid or powdery colouring substances in order to improve the aesthetic features of the articles;the various parts of the plant used to manufacture the said articles must ensure a high level of quality without wastage of material and/or unjustifiable costs. In particular these requirements must be satisfied by the apparatus used commonly called a mix distributor and by the devices for dispensing the colouring substances. A recent example of a mix distributor is described in the patent application WO-A-2004/039547 in the name of the present Applicant and comprises a carriage movable on the top of a tray mould and having a hopper arranged above. The bottom of the hopper is formed by a conveyor called an extractor belt which is inclined downwards and which passes underneath an opening provided at the bottom of a wall of the hopper which on its inner side has a particular shape and extends beyond said wall. A quantity of mix slightly greater than that required for production of a slab is loaded via an upper mouth into the hopper and gradually discharged through said opening so as to be finally deposited inside the mould while the carriage moves above the latter. The object of said inner shape is to favour the dimensional uniformity and agglomeration of the mix deposited inside the mould owing to the fact that the flow of mix inside the hopper is obliged to pass from an initial vertical direction to a direction parallel to the inclination of the extractor belt, extending from the discharge opening at the bottom of the hopper.        
Even though this distributor as a whole is to be considered satisfactory, it requires frequent maintenance operations when (as is usually the case) the plant of which the distributor forms part is used for the mass production of articles. Owing in particular to the already mentioned stickiness, the mix has in fact the tendency to line the extractor belt and form on the abovementioned shaped wall and on the discharge opening of the hopper incrustations which, altering the cross-sectional dimensions of the mix layer discharged onto the extractor belt, could give rise to a non-uniform distribution of the mix. It is therefore necessary to interrupt immediately operation of the distributor as soon as an irregularity in discharging of the mix is noticed, in order to eliminate the incrustations, but obviously these interruptions reduce the productivity of the plant of which the distributor forms part.
As regards the addition of colouring substances to the mixes, from among the most valued and popular aesthetic features of the articles, it is important to mention the so-called veined effect which imitates the appearance of natural stone material.
A first patent in this connection is IT-A-1 273 903, filed on 22 Jul. 1994 in the name of the present Applicant, in which it is envisaged that, before the mix is deposited in a mould, a liquid or powdery colouring substance is added onto its surface and distributed in a discontinuous manner. However, this procedure, not performed in conjunction with the distributor according to the present invention, is unable to ensure over the whole finished product that particular irregular and localized distribution of the said colouring agent which is needed to create a proper veined effect.
The patent application WO-A-03 027 042 describes an alternative method which is able to achieve the desired irregular and localized distribution of the said colouring agent and therefore the veined effect, directly on the support with which a slab is produced, before the vacuum vibrocompaction step is performed. This method has the drawback, however, that the veined effect remains localized on the surface layer instead of being present throughout the thickness of the slab. Therefore, when the slab undergoes polishing of the edges or deep machining before being used, the veined effect is no longer apparent, as would be desirable.
Also worth mentioning is EP-A-1 170 104 disclosing a distributor of powdered products to decorate ceramic tiles comprising a conveyor belt provided with hollow means which is open outwardly to contain parts of the powdered products.